210 Basic Structure of Vertebrates 



is a macula acustica utriculi (Fig. 182). Each of the three ampullae 

 contains a crista acustica. On the main wall of the sacculus is the 

 macula acustica sacculi. Sometimes there is a small macula 

 neglecta near the region of junction of utriculus and sacculus. In the 

 lagena is a macula (or papilla) acustica lagenae. Near the lagena 

 in some amphibians is a papilla acustica basilaris. Corresponding 

 to the great elongation of the lagena to form the mammalian cochlea 

 (Fig. 180G), the sensory area is elongated and elaborated into a complex 



Fig. 181. Sensory cells. (A) Crista cell 

 of ear. (B) Rod-cell of eye. (C) Olfactory 

 cell. (After Fiirbringer. Courtesy, kings- 

 ley: "Comparative Anatomy of Verte- 

 brates," Philadelphia, The Blakiston 

 Company.) 



mechanism, the organ of Corti, which extends throughout the length 

 of the spiral cochlea (see p. 209). 



Cranial nerve VIII divides into as many branches as there are 

 sensory areas, sending one branch to each macula, crista, or other area. 

 In mammals the largest trunk goes to the cochlea. 



The ampullar receptors are probably stimulated by currents in the 

 endolymph caused by movements of the body. The utricular and 

 saccular maculas are probably static in function. The effect which 

 motion of the endolymph may have on the receptor-cells is doubtless 

 augmented by the otoconia and otoliths. The function of the primitive 

 lagena is uncertain, but as it becomes highly developed, as in some 

 reptiles and in birds, it is certainly auditory. The cochlear organ of 

 Corti is the all-important organ of discriminative hearing in mammals. 



Accessory Auditory Structures 



In so far as the ear of fishes may be stimulated by vibrations of the 

 external medium, the vibrations may reach the receptors only by 

 transmission through the tissues of the head. But some amphibians, 

 most reptiles, and all birds and mammals possess important auxiliary 



